With the significant progress of the quality and production technology of hot rolled steel plates and sheets for structure uses in recent years, demand for steel products excellent in plastic deformability has increased more and more, particularly in the field of architecture and civil engineering from the standpoint of anti-earthquake design, and steel plates and sheets are now required to have a high strength, a low yield ratio and a high uniform elongation.
To respond to the requirement, Kokai (Japanese Unexamined Patent Publication) No. 57-16118, for example, discloses a process for producing electric welded tubes of low yield ratio, for oil wells, in which the carbon content is increased to 0.26 to 0.48%, and Kokai (Japanese Unexamined Patent Publication) No. 57-16119 discloses a process for producing high tensile strength electric welded tubes of low yield ratio in which the carbon content is from 0.10 to 0.20%. In either of these processes, electric welded tubes requiring no heat treatment are prepared by producing a hot rolled steel plate or sheet of low yield ratio, and cold working the steel product while the strain amount is being restricted so that the amount of work hardening does not become large. Moreover, Kokai (Japanese Unexamined Patent Publication) No. 4-176818 proposes a process for producing steel tubes or square tubes excellent in anti-earthquake properties by hot working a strainless ferrite-pearlite dual phase structure, controlling the cooling rate after hot working, and heat treating. However, all those processes mentioned above greatly lower the productivity and, in addition, the former processes markedly impair the weldability. Accordingly, those processes currently do not necessarily answer the requirements of the industrial field.
In addition to the disclosures mentioned above, Kokai (Japanese Unexamined Patent Publication) No. 4-48048 and Kokai (Japanese Unexamined Patent Publication) No. 4-99248 disclose techniques for improving the toughness of weld heat-affected region by dispersing oxide inclusions in a steel matrix. The oxide inclusions in the former patent publication are 0.5 .mu.m or less in particle size and have (Ti, Nb) (O, N) composite crystal phases. The oxide inclusions in the latter patent publication are 1 .mu.m or less in particle size and have Ti(O, N) composite crystal phases. The techniques of these patent publications are essentially different from that of the present invention with regard to dispersion phases and objects.
In general, a steel having a higher strength exhibits a higher yield ratio and a lower ductility, and therefore its uniform elongation is lowered. Especially when the steel is cold worked to give round and square tubes, shape steels, sheet piles, etc., its uniform elongation is markedly lowered because of the influence of work hardening caused by work strain.
The present invention has been achieved to solve the problems as described above, and an object of the present invention is to provide hot rolled steel plates and sheets excellent in uniform elongation and having a high tensile strength (at least 34 kgf/mm.sup.2) even after subjecting them to cold working to give round and square tubes, shapes, sheet piles, etc., to such an ordinary degree that the productivity is not lowered.